Conventionally, HIP processing which is a pressing method using a hot isostatic pressing device has been known. In this HIP processing, a workpiece such as a sintered product (ceramics, etc.) or a cast product is processed under an atmosphere of pressure medium gas set at high pressure of several tens to several hundreds MPa, in such a way that a temperature of the workpiece is increased to be equal to or higher than its recrystallization temperature. The HIP processing is characterized in that residual pores in the workpiece can be extinguished. Therefore, this HIP processing has today come to be widely used for industrial purposes in order to improve mechanical characteristics, reduce variations of characteristics, and improve yields.
Incidentally, in an actual production side, speeding-up of the HIP processing is strongly desired. In order to do so, a cooling step which takes time among steps of the HIP processing essentially has to be performed in a short time. Thus, in conventional hot isostatic pressing devices (hereinafter each referred to as an HIP device), an improvement of the coiling speed in a state where the inside of a furnace is maintained in a thermally uniform condition has been considered.
For example, Patent document 1 discloses a hot isostatic pressing device in which a portion of pressure medium gas forming a first circulation flow is allowed by using a fan or an ejector to pass from the lower side of a hot zone to join a second circulation flow and the joined pressure medium gas is cooled and circulated in the hot zone to eliminate a temperature difference generated between upper and lower portions of a furnace in a cooling step, whereby the inside of the furnace is effectively cooled.
In a container of Patent document 1, the low-temperature pressure medium gas is not directly guided into the furnace; therefore, an inner circumferential surface of the container is not excessively cooled. Further, a forcible circulation by means of the ejector can realize a high cooling speed. Furthermore, compared with a case where the fan is provided in the hot zone, the ejector not having the limitation of heat-resisting properties or the like to materials is used; therefore, the furnace structure is not complicated and a cost increase of the HIP device is inhibited.
Patent document 2 discloses a technique in which pressure medium gas in a high-pressure container is removed therefrom and is cooled to be thereafter returned into the container and a cooling step is thereby performed in a short time.
The conventional HIP device provides a quick cooling technique for the purpose of an improvement of productivity, and it can remarkably reduce a cooling time required for cooling from a high-temperature range of from 1000 degrees C. to 1400 degrees C., which is a processing temperature of the HIP processing to a low-temperature range of equal to or lower than 300 degrees C. in which a workpiece can be removed. Specifically, an average cooling speed is generally no more than a few degrees C. per minute in natural cooling; however, a cooling speed of several tens of degrees C. per minute can be attained in the conventional HIP device.
Meanwhile, a solution heat treatment or the like is performed to aluminum alloy casting products or precision casting products of alloys based on nickel. However, these days quickly cooling is performed after the HIP processing; thereby, these heat treatments have been required to be performed successively to the HIP processing. Quick cooling required in such solution heat treatment cannot be performed by a general HIP device, the cooling speed of which is lower; therefore, previously, reheating processing and quick cooling are performed in a different furnace from the furnace for the HIP processing.
Here, the cooling speed required for quickly cooling targeted to aluminum alloy casting products or precision casting products of alloys based on nickel is very high, at least several tens of degrees C. per minute or higher, and a cooling speed of 100 degrees C. per minute or higher may be required depending on thicknesses or materials of workpieces. Such high cooling speed is difficult to be achieved by the conventional HIP device.